[clang] [clang-tools-extra] [clang] Improve nested name specifier AST representation (PR #147835)

[Christian Kandeler via cfe-commits]

ckandeler wrote:

Observation using clangd: In the following construct
```
namespace N { struct Foo{}; }
void f()
{
    using namespace N;
    Foo foo;
}
```
The "arcana" field of the AST node of "Foo" inside the function reports the "QualType" as "Foo", whereas before it was "N::Foo". Is that expected?

https://github.com/llvm/llvm-project/pull/147835